Selecting external stone cladding material

by Katie Daniel | November 24, 2016 10:53 am

By Bill Wright
The exterior cladding of a building is exposed to some of the most extreme and harsh environmental conditions, including tremendous ranges and rates of change of temperatures. As such, determining what stone is suitable can be a daunting task. When choosing an appropriate stone, it is necessary to consider all the physical characteristics required to satisfy the performance criteria of the building design and location.

Factors in choosing the right material
Several common key factors must be considered when selecting a natural stone.

Water absorption rate
Generally, the lower the water absorption rate, the greater the frost resistance, stain resistance, and breaking strength of the stone. These are all desirable qualities for a cladding material. Stone cladding material should be tested according to ASTM C97/C97M, Test Methods for Absorption and Bulk Specific Gravity of Dimension Stone.

Resistance against frost is a function of water absorption rate. Any stone with an absorption rate of three per cent or lower is generally considered to be frost-resistant. Testing to support the application is strongly advised.

High breaking strength
High breaking strength is determined through use of ASTM C99/C99M, Test Method for Modulus of Rupture of Dimension Stone, and ASTM C170/C170M, Test Method for Compressive Strength of Dimension Stone.

Chemical resistance
The stone must have good chemical resistance to prevent deterioration from acid rain and other airborne pollutants. One must also take into consideration the cleaning and maintenance of the natural stone, as well as other parts of the wall assembly, including windows and exposed metalwork. Certain cleaning chemicals and processes can adversely affect the stone, so post-installation treatment of the materials must be considered during the stone selection phase.

Thermal movement and shock resistance
The stone’s rate of expansion and contraction due to temperature changes must be relatively similar to adjacent cladding materials. Significant differences can lead to
excessive stress. There is a difference between thermal shock and thermal movement. Thermal shock refers to the rate and range of temperature fluctuation within short periods. A façade with a southern or western solar orientation exposed to a sudden cool rainstorm can send the temperature of a cladding material plunging within a matter of minutes. Materials should be tested to ASTM C99/C99M, Test Method for Modulus of Rupture of Dimension Stone, and ASTM C170/C170M, Test Method for Compressive Strength of Dimension Stone.

Natural stone options
Natural stone is classified geologically in three categories:

• igneous;
• sedimentary; and
• metamorphic.

Igneous
Igneous rocks are solidified from molten state—examples include basalt and granite. The latter has a primary mineral composition of feldspar and quartz. Some varieties of granite contain trace minerals that can cause discoloration or exfoliation after prolonged exposure to the weather. Granite has a distinct crystalline appearance and is extremely hard, dense, and resistant to scratches and acids. It is a very suitable stone for exterior walls, especially because the density and hardness of granite impart stability and high breaking strength resistance. Granites used in building construction, especially exterior walls, should have a maximum absorption rate of 0.40 per cent by weight according to ASTM standards.

While finishes of stone are primarily an esthetic consideration, it should be noted a thermal finish common on granite induces thermal shock damage to the first 3-mm (¹/8-in.) depth of the stone face, and should be taken into account by deducting this layer when calculating thickness specifications. Other common finishes for external cladding are polished, honed, sandblasted, and bush-hammered. Commercially, granite can be found in hundreds of varieties, differentiated primarily by colour (a function of the mineral composition) and geographic origin.

Sedimentary
Sedimentary stone is created through cementing, consolidating, and crystallizing chemical solutions and biological deposits. Examples include sandstone and limestone—the latter of which has a primary mineral composition of calcite and dolomite. Limestones are geologically categorized as either oolitic or dolomitic, and are commercially categorized as a building stone by density property according to ASTM C 568, Standard Specification for Limestone Dimension Stone.

Metamorphic
A change or alteration of solidified rock by heat, pressure, or intrusion of other minerals results in the metamorphic category, which includes marble, slate, and quartzite.

Marble is a metamorphic stone (originally a limestone) with a primary mineral composition of calcite and dolomite. It has sufficient hardness that it can take a polish finish. Commercially, there are more than 8000 varieties of marble, based on mineral content, colour, and geographic origin.

Marble dimension stone
According to ASTM C503, Standard Specification for Marble Dimension Stone, there are several classifications of marble building stone, but only a few are suitable for exterior cladding. Low-density Category l (12 per cent absorption) and Medium-density Category II (7.5 per cent absorption) are not for exterior use. High-density Category III (three per cent absorption) is only suitable for exteriors with supporting documentation. High-density stones such as Class I−Calcite or Class II−Dolomite are the best suited for exterior cladding and commercial flooring installations.

Class IV−Travertine is a form of limestone that often forms near hot, bubbly, mineral-rich springs. Gas bubbles become trapped and create a pitted surface on the stone. These pitted surfaces can be filled with an epoxy or dust resin. Filling the small holes and pits gives the travertine a more finished look.

The percentage of magnesium carbonate in marble generally determines its strength, colour, texture, and variety. Calcite marbles have 40 per cent magnesium carbonate. Travertine is geologically a limestone, and serpentine (i.e. Class III) is geologically an igneous stone—but as both are capable of taking a polish, they are commercially classified as ‘marble.’

Further, stone industry organizations classify marble into four fabrication, handling, and
working qualities:

• Group A: sound stone with uniform characteristics and favourable working qualities;
• Group B: stone similar to group A; may have some natural faults;
• Group C: stone with variations in working qualities, containing geological flaws, voids, and veins; and
• Group D: contains many of the most highly coloured, veined, and decorative marbles with substantial natural cleavage faults.

While fabrication classifications are not necessarily an indication of the physical properties or durability of stone, it is generally recommended only Group A and Group B marble are suitable for use as external cladding. For more information, design professionals should seek ASTM C1527/C1527M-11, Standard Specification for Travertine Dimension Stone.

The specification process
When possible, it is valuable to solicit the supplier, fabricator, and installer’s participation when one is  specifying stone for exterior cladding. This team can give the best direction for success. The team needs to understand and work within the tolerances provided by the stone fabricator. Fabrication capabilities and manufacturing tolerances will be the key to selecting the stone, sizes available, joint spacing required, and acceptable colour variations (if any).

Tolerances
Whether it is the manufacturing tolerances of the fabricator or the installation tolerances of the setter working on the site, these limitations must be factored into the overall result. Understanding and planning for these required tolerances will help alleviate onsite surprises and headaches.

Manufactured stone can vary in thickness; even smooth-honed and polished finishes can vary ± 0.8 mm (¹/32 in.) from piece to piece in 10 to 15-mm (³/8 to ⁵/8-in.) thick material. Smooth-finished material 15 to 30 mm (⁵/8 to 1 ¹/8 in.) thick can vary ± 3 mm (¹/8 in.) in thickness from piece to piece. More heavily textured finishes (e.g. flamed and heavy mechanical texturing) can vary 5 mm (¹³/64 in.) or more, piece to piece, and still be within the industry standards for acceptable variation.

As with the thickness, there are also acceptable variations in rectangularity and planarity of manufactured stone tiles and panels. The overall squareness of the natural stone panel is defined as the rectangularity. Planarity refers to the flatness of the stone. The ideal working material should be both square and as flat as possible. However, ‘square’ is not clearly defined in the current industry guidelines. It must therefore be a negotiated tolerance between all parties, including the specifier, supplier, quarry, and installer when the stone is ordered from the factory.

The flatness is defined in current Terrazzo, Tile, and Marble Association of Canada (TTMAC) manuals. As an example, it is considered acceptable if a stone tile up to a 600 x 600-mm (24 x 24-in.) size has a variation no more than 0.8 mm across 600 mm; for larger cut-to-size pieces, a variation of less than 1.5 mm (¹/16 in.) across 1200 mm (48 in.) is considered ‘flat.’

Installers also have a set of industry standards and tolerances to which they adhere. These standards govern lippage from piece to piece (for smooth finished, 0.8 mm is tolerable) and the joint width spacing (±25 per cent of the specified dimension), to name a couple.

What happens when the plan is not made or understood? Recently, this author’s firm supplied a stone with manufacturing tolerances of ±0.5 mm (¹/64 in.) for both rectangularity and planarity sizing. The installers had agreed to a 1-mm (¹/32-in.) joint spacing as requested by the owner. The tiles were requested to be square-edged with no bevelled edges. The installer has had huge issues with chipping along the edges when the stones were placed so tightly together. These chipping problems were caused by sharp corners knocking together due to the joints being too tightly spaced. Also as a result, the tight joints the evenness of joint spacing started to creep, resulting in unsightly misaligned joints.

This was made even more problematic when the installers failed to use gloves when handling the stone for installation. The natural oils from their hands left marks requiring a special cleaning and spot removal process. Overall, it was a very expensive lesson for the installers. To be successful, this type of installation requires at minimum a 2-mm (⁴/5-in) wide joint space for stone, and installers should always wear gloves when handling natural stone.

It is recommended to follow all the TTMAC industry manuals’ including 093000 Tile and Stone Installation Manual, the Dimensional Stone Guide, and Hard Surface Maintenance Guide. Additionally, the ongoing co-operation of all concerned parties is paramount for a successful outcome.

Conclusion
Understanding all the parts that have to work together for success is not an easy or simple task. Since many players, including a stone consultant, need to be on this team, working with experienced stone-installers and partnering with educated suppliers will make the selection, supply, and execution process better. When comparing the various stone options, a basic understanding and review of test data will provide the information and understanding of which stone will meet the desired expectations. Suppliers and stone installers are available to help with this review. They will be your rock for when you are in a hard place.

Bill Wright is a technical specialist with Stone Tile International, and is currently heading its training and education department. He brings three decades of experience in all aspects of the tile and stone industry, from installation and manufacturing to architectural/contractor sales and management. For the past decade, Wright has served as a director of the Terrazzo, Tile, and Marble Association of Canada (TTMAC), where he was a regular contributor to the last three editions of the TTMAC Installation Manual. He can be reached at bwright@stone-tile.com[1].

Source URL: https://www.constructioncanada.net/selecting-external-stone-cladding-material/

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